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Nowhere to hide from SOHO

10 March 2006

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The Sun is filled with many kinds of sound waves caused by the boiling motion of gas in its surface layers. SOHO’s Michelson Doppler Imager (MDI) instrument traces these ‘sound waves’ reverberating through the Sun to build up a picture of the far side. The MDI uses a far-side imaging method which compares the sound waves that emanate from each small region on the far side, with what was expected to arrive at that small region from waves that originated on the front side.

Credits: ESA/NASA

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This diagram shows some sample sound wave paths connecting the front or Earth side of the Sun with the back or far side. Sound waves are generated by the boiling motion of gas in the Sun's surface layers, called convection.

When country-sized convection cells called 'granules' cool and fall back into the interior they cause sound waves to move into the Sun in all directions. This diagram shows the path of some of the waves from one of these cells starting on the front side.

As the waves move into the Sun they get bent out back to the surface because the sound speed is higher deeper into the Sun. When they get to the surface they get reflected back into the Sun. When they get reflected, the surface layer moves and scientists are able to measure that movement.

Credits: Stanford University

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